The high pressure feeder, or transfer device, is one of the most basic and important components of the Kamyr continuous pulping system. The high pressure feeder is used to transfer steamed wood chips in a liquid (typically white liquor) at low pressure to the top of the continuous digester, at high pressure. A typical high pressure transfer device comprises a rotor having through extending pockets disposed in first and second sets spaced along the axis of rotation of the rotor a housing, a screen, and high and low pressure pumps. The rotor pockets each have opposite end openings which function as both inlets and outlets depending upon the angular position of the rotor, and the pockets of one set (typically two) are offset from those of the other. The housing encloses the rotor and has an exterior periphery with first through fourth ports for each set disposed around the exterior periphery for registry with the inlets to and outlets from the pockets. The first and third ports are opposite, and the second and fourth ports are opposite, and the first and second ports may be adjacent in the direction of rotation.
In a conventional high pressure feeder screen means are disposed in the third port of each set for screening particles above a predetermined size out of the liquid passing through the third port, and a single low pressure pump is connected to a single conduit from the third ports to provide the suction for sucking liquid through the third port. A high pressure pump is operatively connected to the second port to provide the flow of liquid under high pressure through the second port. Normally the first port is on the top, and the third port on the bottom, the first port connected to the chips chute, and the fourth port connected to the top of the digester.
While conventional high pressure feeders have functioned very well over the decades they have been in use, there have been relatively few substantive changes to the high pressure feeder over time. It has been known that the filling efficiency of the high pressure feeder is approximately 50 to 60% on some chip furnishers that is significantly lower than is desired, but to date no significant inroads have been made in substantially increasing that efficiency since the source of the lack of efficiency has not been understood.
To a large extent, the efficiency of the high pressure feeder is dictated by its ability to obtain the chip chute circulation which carries the chips from the chute into the pockets of the rotor. The chip chute circulation is throttled on the suction side of the chip chute circulation pump by the pressure drop across the screen at the third port.
According to the present invention, for the first time it is understood why this pressure drop occurs. After extensive testing, according to the invention it has been found that as the consecutive pockets open in the different sets of the rotor, as the chips and liquid are flowing into the most open (first opened) pocket because it is connected to the same suction source as the pocket in the other set that is just opening, there is little pressure to start flow into the second opening pocket. Thus, the second pocket will not really start to effectively fill until the first pocket has moved to a position where it is starting to significantly close. Thus the feeder suction is throttled. Therefore if the feeder is rotated faster, the time available to fill the feeder goes down, and thus the high pressure transfer device is forced to run at a lower filling efficiency than desired to get enough flow.
In addition to identifying the source of the relatively low high pressure transfer device efficiency, according to the present invention the problem has been solved by providing a distinct source of suction for the third pocket of each set to suck liquid through the screen means of that third port so as to enhance the filling efficiencies of the pockets at any given speed of rotation of the rotor. There are two presently contemplated major ways in which the separate suction sources can be provided. In a first way, first and second elongated conduits extend from the separate third ports (the third port associated with each set), and the conduits are connected to a common low pressure pump. In a second embodiment, the distinct elongated conduits are connected to separate pumps. When the conduits are connected to separate low pressure pumps, it may be necessary or desirable to provide a mechanism to prevent water hammer or pump cavitation. This may be provided by utilizing a cross connection having an orifice therein between the elongated conduits, just before the low pressure pumps.
The invention also contemplates a method of transferring wood chips in liquid through the high pressure transfer device to boost the flow rate thereof. That method includes the conventional steps for feeding wood chips utilizing a high pressure transfer device, but in which suction is supplied separately to the third port of each set, so as to enhance the filling efficiency of the pockets for a given speed of rotation. This may be accomplished by connecting the third ports through elongated conduits to a single pump, or to separate low pressure pumps, with a water hammer preventing connection therebetween, as described above.
It is the primary object of the present invention to provide for enhanced efficiency of a conventional high pressure transfer device, particularly for use in a method of boosting the flow rate of a slurry of wood chips. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.